1. Field of the Invention
The present invention relates to an improved sheet metal hemming process.
2. Background Art
Hemming is a production process for joining an outer panel to an inner reinforcement panel. Conventional hemming processes are accomplished by bending a peripheral edge of the outer panel back onto the inner panel. A multiple step process is used to form a hem. In a first step, a peripheral edge of an outer panel is formed to extend substantially perpendicularly relative to the body of the outer panel. In a second step, an inner panel is assembled to the outer panel with a perimeter flange being inserted inside the peripheral edge of the outer panel. In the third step, the peripheral edge of the outer panel is pre-hemmed. Pre-hemming may be performed by forming the peripheral edge to an acute angle of approximately 45° to extend inwardly over the perimeter flange of the inner panel. Alternatively, the edge may be pre-formed in two steps to 60° and then 30°. In a final step, the peripheral edge is formed to extend parallel to the body of the outer panel and engage the perimeter edge of the inner panel.
Hems in highly visible areas such as hoods and doors are key to assessing a vehicle's overall craftsmanship. This is especially true when two closure panels are located adjacent to each other as in the case of a gap formed between two doors or between a hood and a fender. The radius of the hem visually impacts the appearance of the gap, or margin, between the two panels. Larger hems tend to make the margin look larger, while smaller hems produce a crisper cut line and smaller perceived margin. In the prior art, as the thickness of the inner panel increases, the hem radius increases. This, in turn, increases the perceived gap between matched panels. In conventional hemming processes, the final hem radii is a function of the thicknesses of the inner and outer panels. With a conventional hem, the radius is equal to one-half the inner panel thickness plus the outer panel thickness.
Recent developments in the field of hem forming have led to the development of reduced radius hems that improve the appearance of the fit of adjacent panels by reducing the perceived margin between adjacent panels. One example of such a hemming method is disclosed in U.S. application Ser. No. 10/063,757, filed in the name of Samant et al., and assigned to Applicant's assignee. The Samant application describes a method of manufacturing a hem with a reduced radius hem. The method and apparatus discussed in the Samant application produces a hem with a radius sharper than half the total stack height of the inner and outer panels and can significantly improve the craftsmanship of closure panels.
A marriage gap is defined as the space between the outer edge of the perimeter flange of the inner panel and the bend of the peripheral edge of the outer panel. The marriage gap is provided to avoid interference between the two panels. Interference between the panels occurs when, during the pre-hemming pass, the outer panel peripheral edge is being formed into an acute angle to extend over the perimeter flange of the inner flange and improperly comes into contact with the inner panel.
Referring to FIG. 1, a prior art hem flanging operation is illustrated in which an interference occurs between a relatively thick inner panel I and the peripheral edge P of the outer panel O during pre-hemming. A hemming tool R is shown contacting the peripheral edge P of the outer panel O during a pre-hemming pass. A top corner C of the inner panel I interferes or makes contact with the inner surface of the peripheral edge P. This interference may cause distortion in the hem and limit reduction of the hem radius. Interference may cause splitting or tearing of the hem. The marriage gap can be increased to prevent the interference to a limited extent. For conventional hems, the marriage gap is generally approximately 2 millimeters. If the marriage gap is too large, the hem may be weakened and the edge of the panel will become prone to bending or deformation.
Recently, lightweight inner panels have been introduced that are cast or otherwise formed of magnesium, aluminum and other alloys, polymers, or similar materials. These panels are generally thicker than conventional metal inner panels. Conventional steel panels are approximately 0.7-0.8 mm. Conventional aluminum panels range between 0.9 and 1.2 mm. Inner panels made of other materials may be of greater thickness. For example, a cast magnesium alloy inner panel may be 3 to 4 times the thickness of a steel inner panel. Such thick inner panels increase the radius of hems which increases the apparent gap size and adversely effects overall quality and craftsmanship.
The use of thick inner panels creates unique hemming challenges. For thicker inner panels, a 2 millimeter marriage gap is not always sufficient to avoid interference between the panels when using a reduced radius hem. A conventional solution to eliminate interference is to increase the marriage gap. Increased flange length is required to cover the larger marriage gap. This solution may be acceptable in flat-straight areas of the panel but can cause problems in hems on curved edges or surfaces.
Short flange lengths must be provided in areas where there is either cut line or surface curvature. In these areas, splits in either the edge or flange can be caused due to the stretching or compressing of the flange during the bending operation. A conventional solution to solving hemming problems in such areas is to reduce the length of outer panel flanges to mitigate this problem. It may be necessary to reduce outer panel flange lengths to 6 millimeters or less to accommodate cut lines and surface curvatures.
Thick inner panels when hemmed with a reduced radius hemming process may result in edge run out. Edge run out is an unacceptable condition that occurs if the hem edge is not long enough to cover the marriage gap. This problem is particularly difficult with hems in panels having curved cut lines and surface contours.
There is a need for an improved reduced radius hem and hemming process that can accommodate thick inner panels while maintaining the size of the marriage gap and minimizing flange lengths in critical contoured areas. The disadvantages and shortcomings of the prior art are addressed by this invention as summarized below.